svoid setOneLineFileInfoField(File f, S field, O value) {
  File infoFile = associatedInfosFile(f);
  LS lines = loadTextFile(infoFile);
  S newLine = value == null ? null : field + ": " + str(value);
  S line = firstStartingWithIC(lines, field + ": ");
  if (eq(line, newLine)) ret;
  saveTextFile(infoFile, lines(replaceAddOrRemove(lines, line, newLine)));
}